Cis-butene oxidation



Patented Jan. 9, 1951 CIS-BUTENE OXIDATION l Leland K. Beach, Mountainside, N. J assignor to Standard Oil Development Company, a corporation of Delaware No Drawing. Application January 3, 1949, Serial No. 69,051

Claims. (01. 260--533) This invention relates broadly to the catalytic oxidation of hydrocarbons to yield polybasic organic acids and anhydrides and more specifically to the vapor phase catalytic oxidation of cis-2- butene to give increased yields of maleic acid.

It is to be understood that the term maleic acid as used throughout this specification and claims includes also maleic anhydride, which is frequently obtained as the predominating product as well as mixtures of the acid and the anhydride. It is to be noted that the term crude maleic acid is used throughout the specification and claims to include both maleic acid and anhydride and small amounts of various other acids such as are normally obtained to a lesser extent in vapor phase oxidations of thebutenes.

It has been known that both l-butene and 2- butene can be oxidized to maleic acid in approximately 20-25 mole per cent yield. The ordinary 2-butene as obtained commercially, for instance, from various refinery processes and as a by-prodnot in mixtures of other related hydrocarbons consists of a mixture of cis-2-butene and trans-2- butene with the trans isomer predominating in mixtures from normal sources. The two structures result from space isomerization about the olefinic double bond and the most probable structures may be represented by the following formulas:

The two isomers of 2-butene show a diiference in boiling point, the cis compound boiling at +3.'I2 C. and the trans at +0.88 C. It is believed most probable that the isomer known as the cis isomer has the two methyl groups on the same side of the double bond while the trans isomer has the methyl groups positioned on opposite sides of the double bond.

It has now been discovered that a very surprising and unexpected difference exists in the conversion yields to maleic acid of the two isomers under catalytic oxidation conditions.

In fact, the yield of crude maleic acid obtained from a feed stock high in cis-2-butene content is much higher than either that obtained when trans-2-butene or various mixtures of the two isomers are used. This difference is unobvious from the known differences in physical and chemical properties of such compounds. The increased yield of crude poly basic acids from cis- 2-butene over that from trans-2-butene is very substantial and of the order of 40 per cent. Thus most favorable.

from a final yield standpoint, there is a distinct advantage in using as the oxidation feed stock a cut containing as high a percentage of cis-2- butene as possible. Obviously, the best yields are obtained when the pure cis- 2-butene is used without any other butene isomers present. However, from a practical standpoint, the use of a butene feed stock having at least a per cent concentration of cis-2-butene is most desirable for obtaining high yields of crude maleic acid. It is, furthermore, very desirable to oxidize a hydrocarbon fraction having as low a concentration o a trans-2-butene as it is practically possible to obtain.

The 2-butene fraction is almost invariably obtained as a mixture of the cis and trans compounds. source which may be used.

Petroleum fractions containing unsaturated hydrocarbons of the C4 range containing cis-2; butene may be obtained by steam cracking volatile petroleum stocks, such as naphtha cuts and gasolines. Temperatures of the order of 1000- 1600 F. and pressures of 1 to 10 atmospheres are The charging stock is preferably diluted with up to mole per cent of steam in the cracking zone in order to restrict carbonization.

The hot cracked products issuing from the cracking zone are quenched. In processing these qenched cracked petroleum products, the aromatic naphtha, together with the lower boiling hydrocarbons including part of the C4 fraction and steam, are removed overhead as volatiles to separate them from the higher boiling cracked products which are quenched to a liquid state. The volatiles may, if desired, be combined with the higher boiling liquid quenched products and subjected to controlled fractional distillation treatment from which a C4 stream may be separated. This C4 stream will contain substantial amounts of butenes as well as butadienes and C4 paraffins. The boiling points of the components most likely to be present are given below:

A fraction high in cis-Z-butene content, that is, having at least a 75% cis-2-butene concentra- In general, this is true regardless of the tion, can then be obtained from this C4 fraction. The butadienes can be readily separated by a selective copper salt extraction to remove substantially all of the butadiene in the extract.

Distillation of the resulting fraction in a fractionating column having 20-40 plates, following extraction to remove the butadienes, can be employed to give a C4 hydrocarbon fraction boiling within a relatively narrow range of approximately to +5 C. and containing chiefly cis- Z-butene, trans-2-butene, l-butene, and isobutene with relatively small amounts of butadiene and the C4 parafllns, including both butane and isobutane. The isobutene, which gives at best only very inferior yields of the crude maleic acid, is preferably removed prior to any catalytic oxidation step. This can be done quite satisfactorily by an acid adsorption operation. The removal of isobutene is carried out by contacting the mixed C4 hydrocarbon stream with sulfuric acid of approximately 65% concentration. Operating thus. the isobutene is essentially completely removed by absorption into the extracting acid from which it may subsequently be recovered by desorption, if desired. The C4 olefins, including l-butene and both cis and trans z-butene, together with minor amounts of butadienes and butanes are not appreciably affected by 65% sulfuric acid. This fraction is then subjected to a fractionation, or some other convenient method, for the separation of a fraction having a content of cis-Z-butene of at least 75%. This separation of cis-2-butene is readily carried out by the use of a super-fractionation tower having about 100 plates. A fraction consisting, to a major extent, of the cis isomer, can be removed from a lower portion of the tower since, as can be noted from the table of boiling points elsewhere included, the ois-2-butene is the highest boiling component of all those commonly present in C4 olefin fractions.

Higher temperatures. in general, tend to favor the formation of the cis-2-butene as compared to both l-butene and trans-2-butene in the equilibrium mixture. Isomerization agents such as sulfuric acid may be used to advantage to effect reformation of the equilibrium mixture contain- 'ing appreciable quantities of the cis-2-butene isomer after the mixture has been depleted of the cis isomer by fractionation In general, concentrations of sulfuric acid of the order of 70% will cause appreciable conversion of a mixture of l-butene and trans-Z-butene to the cis-2-butene, although some care must be exercised both in control of acid concentration limits and in temperature to avoid excessive polymerization.

The oxidation may be carried out by any convenient procedure for vapor phase catalytic oxidations. Special kinds of construction materials are not required since corrosion troubles are negligible. Iron or steel reactors have been found quite satisfactory. The catalyst may be employed in a fixed bed or it may be of the moving bed or fluid flow type such as have been found especially advantageous in other catalytic operations.

A transfer line reactor may offer special advantages. In this type of reactor the finely divided, fluidized oxidation catalyst is introduced into the bottom of the reaction zone by means of a stream of inert gaseous carrying medium. The catalyst is carried upward through the reaction zone into which the hydrocarbon feed is also introduced. Oxygen or air may also be introduced into the reaction zone either with the catalyst or separately. The oxidized products containing the crude maleic acid are removed as the outlet stream from the upper portion of the reactor. After passing through the reaction zone, the catalyst particles are preferably exposed to a stripping treatment, for example, by steam to remove products and unreacted feed. The stripped catalyst is passed continuously through a standplpe to the lower portion of the reactor and thence recycled through the reactor zone. This arrangement gives a valuable advantage in carrying out exothermic oxidations as heat can be removed from the catalyst while it is outside the heat generating reaction zone.

As catalysts for this reaction there may be used any of those commonly employed for catalytic oxidations of hydrocarbons to carboxylic compounds. Among the best-known are the vanadium-containing catalysts. These may be vanadium oxide alone or they may be mixtures of vanadium and other active oxides, as for example, molybdenum oxide. Suitable promoters such as sodium or potassium sulfate may be added to these catalysts.

These catalysts may be of the supported type, using preferably as supporting agent a material which is inert to the reaction conditions. The catalytic material may be employed without support, thus simplifying separation steps. The catalyst may be of the formed type. The catalyst may be in a finely divided state as, for instance, the fluidized catalysts. -An especially valuable type of catalyst for use in such apparatus as the transfer-line reactor is a catalyst having the shape of microspheres. These spheres may be produced by heating the catalyst material to fusion followed by treatment to give the catalytic material the form of microspheres. These microspheres can be used satisfactorily for longer periods of time since they are not subject to the attrition and grinding dimculties inherent in the use of other forms of catalyst particles. Problems created by dust formation are also reduced to a minimum.

The oxidizing gas which is used may be any oxygen-containing gas. Pure oxygen may be used with or without a diluent such as steam. Air is a very convenient and useful oxidizing mixture. Synthetic mixtures can also be used in which omrgen is admixed with an inert gas such as nitrogen.

It is also possible to carry out the oxidation in the absence of an oxidizing gas. Certain catalysts, in particular those of vanadium oxides, may be employed as oxygen carriers which can be en riched with oxygen in a zone completely separated from that in which reaction occurs. This type of reaction can be carried out in a number of ways. It is particularly well-adapted for use in a modified transfer line type reactor.

In cases where an oxidizing gas is used, it should preferably be present in a substantial excess in the feed mixture over the hydrocarbons being oxidized. Usage of this kind of excess tends to decrease formation of tarry byproducts by over-oxidation and decomposition. A preferred method for operation at maximum efficiency employs a feed having hydrocarbon concentrations of 0.50 to 2.00 mole per cent. Concentrations as high as 3-4% of hydrocarbon in the feed stock to the oxidation chamber may be used.

The temperatures which are best employed in carrying out the oxidation reaction are in the range of 300-500 C. The temperature within the catalytic zone, that is tov say, the catalyst temperature, should be high enough to effect the desired conversion of the feed to dibasic acids but not so high as to give excessive combustion resulting in loss of product, contamination of the catalyst, and impurities which give expensive and unnecessary separation dilficulties. An optimum temperature has been found to be 340-350 C. This optimum depends somewhat on type of apparatus and catalyst, composition of feed, catalyst contact time, and other variables of the process. The temperature used will depend somewhat on the amount of conversion of hydrocarbon to crude acid desired per pass of the feed stock.

The mixture of oxidizing gas and hydrocarbon feed should be contacted with the catalyst at such 2 rate as to be practical for commercial operation, allowing sufiicient time for such conversion to the crude diacid product as is most desirable. It has been found preferable to operate the process at a catalyst contact time of about 0.1 to 1.0 second. Any feed stock which is un-oxidized may be recovered from the exit gases and recycled to the reaction zone.

The invention will be more fully explained by reference to the following example.

Example A feed stock composed substantially of cis-2- butene in 1 mole per cent concentration in air was contacted in a steel reactor with an oxidation catalyst of the approximate composition of 12% V205 and 4% M00: supported on corundum and promoted with a small amount of sodium sulfate.

The over-all feed rate to the reactor of about 4000 volumes of air per volume of catalyst per hour corresponds to a contact time of about 0.3 second. The bath temperature of the reaction vessel was about 340-350 C. The exit gases from the reaction zone were condensed and the crude maleic acid product recovered. A substantial yield of acid was obtained. The product was readily identified as maleic acid by its neutral equivalent.

A similar catalytic oxidation experiment was carried out using as feed stock a hydrocarbon mixture consisting substantially of trans-2- butene and gave a much lower yield of acidic products.

It was found that, by operating in the abovedescribed manner using as feed a fraction containing substantially pure cis-2-butene, a 40% improvement in yield of crude maleic acid was obtained over that which resulted when substantially pure trans-2-butene was oxidized in a com parable manner.

What is claimed is:

1. A process for the preparation of maleic acid which comprises oxidizing substantially pure cis- 2-butene with air in the presence of a mixed vanadium oxide-molybdenum oxide catalyst at temperatures of 340-350 C.

2. A process according to claim 1 in which the concentration of cis-2-butene in air is about 1 mole per cent and the catalyst contact time is about 0.3 second.

3. A process for the preparation of maleic acid REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,097,904 Walters Nov. 2, 1937 2,260,409 Slotterbeck et al. Oct. 28, 1941 2,390,934

Gregg Dec. 11, 1945 

3. A PROCESS FOR THE PREPARATION OF MALEIC ACID WHICH COMPRISES OXIDIZING CIS-2-BUTENE SUBSTANTIALLY FREE TRANS-2-BUTENE WITH AN OXYGEN-CONTAINING GAS IN THE PRESENCE OF A VANADIUM OXIDE CATALYST AT TEMPERATURE OF 300* C. 